Methods and apparatus for maintaining equilibrium pressure in a container

ABSTRACT

A system for maintaining a gas in equilibrium with a liquid stored in a container with the gas. The system includes a sensor to detect an equilibrium condition within the container, and to generate a first signal in response. A source gas connection allows a pressurized source gas to flow to the container, and a source gas control valve controls flow of the source gas to the container. The system further includes a controller configured to receive the first signal, and also to determine when fluid is removed from the container. The controller is further configured to operate the source gas control valve to allow the source gas to flow into the container when the controller determines that fluid has been removed from the container. The controller allows the source gas to flow into the container until an equilibrium pressure is reached which is a function of the first signal.

FIELD OF THE INVENTION

[0001] The invention claimed and disclosed herein pertains to methods and apparatus for maintaining equilibrium pressure in fluid containers configured to contain a liquid having a dissolved gas, with a vapor space above the liquid.

BACKGROUND OF THE INVENTION

[0002] In many instances it is desired to provide a liquid which contains a selected volume of dissolved gas in the liquid. One common example is carbonated beverages, although many other examples are also known. In the carbonated beverage example, it is desired to maintain a selected volume of dissolved carbon dioxide (CO₂) within the liquid beverage prior to and during the beverage being dispensed from a storage container, such as a bottle or a keg. The volume of dissolved CO₂ in the beverage is selected to provide the beverage with an effervescence once it is dispensed from the container. If an insufficient volume of CO₂ is present in the dispensed beverage, it will appear “flat”. On the other hand, the volume of dissolved CO₂ should be selected so that it will not produce excessive foaming of the beverage when the beverage is dispensed from the container.

[0003]FIG. 1 depicts a side elevation schematic diagram of a prior art apparatus 10 for maintaining pressure in a liquid storage container 12. The container 12 is configured to hold a liquid 14, and has a vapor space 16 above the liquid wherein a gas is present in vapor form. Typically, the vapor in the vapor space 16 is maintained at a pressure above atmospheric pressure (i.e., the pressure outside of the container 12) so that when the fluid 16 is dispensed from the container 12, the dissolved gas will begin to come out of solution. (This is the effect observed when a carbonated beverage such as beer is dispensed from a keg into a glass for drinking.) The container 12 has a fluid outlet 20 which is connected to a valve 22, allowing the fluid to exit the container 12 via the spigot 24. The container 12 is also provided with a gas connection 18 which is connected to a source of pressurized gas 28 via a gas supply line 26. The source of pressurized gas is maintained at a pressure which preferably exceeds the highest desired pressure of the gas within the container 12. Disposed within the gas supply line 26 is a pressure regulator 30. The pressure regulator 30 is set to maintain a preselected pressure “P” within the container. That is, as the pressure within the container falls, the pressure regulator 30 opens to allow gas from the pressurized gas source 28 to enter the container 12 until the pressure within the container 12 is equal to the preselected pressure “P”. In this way, the vapor pressure of the gas within the container 12 is always maintained at the preselected pressure, as established by the regulator 30.

[0004] The prior art apparatus described immediately above suffers from the primary shortcoming that it does not account for changes in temperature within the container 12. For example, if a beverage in a container, such as container 12 is stored at a temperature of 44° F. (approximately 6.7° C.), and is provided with a gas, such as carbon dioxide, and the desired-gas-volume-to-liquid-volume ratio is 2.5 (based on conditions at standard temperature and pressure), then a preferred pressure within the container is approximately 14.4 psig (approximately 1.01 kg/cm²). However, if the temperature should change to 34° F. (approx. 1.1° C.), then the pressure required to maintain the desired gas-to-liquid volume ratio of 2.5 is 9.2 psig (approx. 0.65 kg/cm²). Thus, if a pressure regulator is set to maintain the gas pressure at 14.4 psig, then as the temperature of the liquid in the storage container changes, the gas volume ratio will vary from the preferred, selected ratio. In the example just given, if the gas pressure is maintained at 14.4 psig, and the temperature of the liquid drops to 34° F., then the gas-to-liquid volume ratio will be in excess of 2.5, resulting in undesirable excessive foaming as the beverage is dispensed from the container 12.

[0005] What is needed then is a method of storing a liquid in a container which allows a preselected ratio of gas to remain dissolved in the liquid, regardless of changes in temperature of the liquid.

SUMMARY OF THE INVENTION

[0006] One embodiment of the present invention provides for a system for maintaining a gas in equilibrium with a liquid stored in a container with the gas. The system includes a sensor configured to detect an equilibrium condition within the container, and to generate a first signal in response thereto. A source gas connection is configured to allow a pressurized source gas to flow to the container, and a source gas control valve is configured to control the flow of the source gas to the container. The system further includes a controller configured to receive the first signal, and also to determine when fluid is removed from the container. The controller is further configured to operate the source gas control valve to allow the source gas to flow into the container when the controller determines that fluid has been removed from the container. The controller is also configured to allow the source gas to flow into the container until an equilibrium pressure is reached which is a function of the first signal.

[0007] In one variation on the embodiment of the invention just described, the sensor is a pressure sensor, and the first signal is a pressure signal. In this case, the equilibrium pressure is essentially equal to the pressure signal at a time prior to a time when the controller determines that fluid has been removed from the container.

[0008] A second embodiment of the present invention provides for a method of maintaining an equilibrium pressure between a gas and a liquid within a container. The method includes providing a container containing a fluid comprising a liquid and a gas in equilibrium, and determining when fluid is removed from the container. A pressure of the gas in the container is measured at about the time, or just before, it is determined that fluid has been removed from the container. A make-up gas is then added to the container until the pressure in the container is approximately equal to the measured pressure.

[0009] These and other aspects and embodiments of the present invention will now be described in detail with reference to the accompanying drawings, wherein:

DESCRIPTION OF THE DRAWINGS

[0010]FIG. 1 is a side elevation schematic view of a prior art system for storing a liquid having a gas dissolved in the liquid.

[0011]FIG. 2 is a schematic diagram depicting a system for storing liquids having a gas dissolved in the liquid, in accordance with one embodiment of the present invention.

[0012]FIG. 3 is a flowchart depicting a series of steps which can be performed to implement a method of storing a liquid, in accordance with the present invention.

[0013]FIG. 4 depicts a first time-pressure chart illustrating the operation of a system of the present invention.

[0014]FIG. 5 depicts a second time-pressure chart illustrating the operation of a system of the present invention.

[0015]FIG. 6 illustrates a look-up table which can be used to determine a gas-liquid equilibrium pressure within a container as a function of the temperature within the container for selected gases.

[0016]FIG. 7 depicts a third time-pressure chart illustrating the operation of a system of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

[0017] The present invention provides methods and apparatus for maintaining a desired gas-to-liquid volume ratio between a gas and a liquid stored in a container. More particularly, the methods and apparatus of the present invention maintain the volume ratio of absorbed gas to liquid, regardless of changes in temperature of the gas and the liquid in the container. The present invention is based on determining an equilibrium condition that exists within the container prior to fluid (either a portion of the gas or a portion of the liquid) being removed from the container, and subsequently adding make-up gas to the container to reestablish the observed equilibrium condition which existed prior to the fluid being removed from the container. Preferably, the equilibrium condition 11 is determined by measuring the pressure of the gas in the container at a first time which is prior to the time when the fluid is removed from the container. The make-up gas is then added to the container to bring the pressure of the gas in the container back to the measured pressure. Thus, the equilibrium pressure is reestablished, or “maintained” in an overall sense, in the container.

[0018] Within a closed container which contains both a gas and a liquid, an equilibrium condition will exist between the gas and the liquid such that the net difference between gas evolved from the liquid and gas absorbed by the liquid is zero. As the temperature of the gas/liquid fluid rises, the pressure of the gas above the liquid will also rise, thus maintaining the equilibrium state. Likewise, as the temperature of the gas/liquid fluid drops, the pressure of the gas above the liquid will also drop, thus again maintaining the equilibrium state. It is only when one of the fluids—either a portion of the gas or a portion of the liquid—is removed from the container is there a possibility that the equilibrium condition can (and generally does) change. Specifically, removal of any of the gas from the container without removal of any of the liquid will lower the overall gas-to-liquid ratio in the container and also lowers the pressure within the container, thus allowing more gas to evolve from the liquid, thereby changing the volume ratio of the gas within the liquid. Likewise, removing any of the liquid from the container without removing any of the free gas will allow the pressure within the container to drop, again allowing more gas to evolve from the liquid. Accordingly, to maintain equilibrium after any of the fluid has been removed from the container, make-up gas is added back into the container to reestablish the equilibrium condition that existed prior to removal of the fluid from the container.

[0019] The present invention also provides a method for determining when a sufficient amount of make-up gas has been added to the container to reestablish equilibrium after fluid has been removed from the container. In general, this is accomplished by determining the state within the container at equilibrium, and then reestablishing that state after fluid has been removed from the container. One specific way in which this can be accomplished is by determining the pressure within the container at a first time which is prior to a second, later time at which fluid is removed from the container. If the temperature of the fluid in the container remains constant, then the state at the first time (the time at which the fluid is at equilibrium) can be reestablished by adding make-up gas to the container until the pressure within the container is essentially equal to the pressure determined at the first time. This method can be further complemented by measuring the temperature of the liquid in the container both at the time the pressure is first measured, and the time the make-up gas is added. If the temperature is found to have changed, then the amount of make-up gas added to the container can be corrected for temperature. I will now describe these and other aspects of the present invention with respect to the attached drawings, which depict exemplary methods and apparatus for practicing the present invention. I will first describe an apparatus in accordance with the present invention, followed by a description of the operation of the apparatus, and I will then describe methods in accordance with the present invention.

[0020] Turning now to FIG. 2, a system 100 in accordance with one embodiment of the present invention is depicted in a schematic diagram. The system 100 is a system for maintaining equilibrium between a liquid “L” and a gas “G” in a container 112. The gas “G” and the liquid “L” together constitute a “fluid” (as the term will be used herein), even though the gas “G” is separate from the liquid “L”. Further, when I refer to the “gas” herein, I mean the free gas “G” which is present over the liquid “L”, and not the gas which is dissolved in the liquid (unless expressly stated otherwise). An example of the liquid “L” is a beverage, such as beer, a soft drink or water, and an example of a gas “G” is carbon dioxide, although the invention should not be considered as being restricted to any specific type or groups of liquids or gases. It is generally assumed that the fluid in the container 112 is maintained at a pressure which exceeds the atmospheric pressure “A” outside of the container, although again, this is not a requirement. The container 112 has a liquid outlet connection 146 which is configured to be connected to a fluid outlet line 142. The fluid outlet line is in turn connected to a liquid outlet valve 122 to allow liquid to be removed from the container. The liquid outlet valve 122 can be, for example, a tap or a faucet, as well as other types of valves, including a ball valve, a globe valve, a gate valve, and a plug valve. The outlet valve 122 can be selectively actuated either manually or automatically to allow liquid “L” to flow from the container 112.

[0021] The system 100 includes a source gas connection configured to allow a pressurized source gas 128 (the “make-up gas”) to flow to the container 112. The gas source 128 can be, for example, a canister or bottle of pressurized gas. Preferably, the make-up gas is the same type of gas as is the gas “G” in the container 112. The source gas connection can be the gas supply line 132, 136 defined by a first end configured to be placed in fluid communication with the pressurized source gas 128, and a second end configured to be connected to the container 112, as for example at the source gas connector 144.

[0022] The system 100 further includes a source gas control valve 134 configured to control the flow of the source gas 128 to the container 112, as will be explained further below. In the example shown, the source gas control valve 134 is disposed within the gas supply line between the first portion 136 and the second portion 132. However, the source gas control valve 134 can be connected directly to the container 112, or directly to the pressurized source gas supply 128.

[0023] The system 100 also comprises a sensor configured to detect an equilibrium condition within the container 112, and to generate a first signal in response to the detected condition. One such sensor is the pressure sensor 124 which is depicted as being connected to the gas supply line 132. The pressure sensor 124 is configured to generate a pressure signal in response to detecting the pressure of the gas within the line 132. In this way, the pressure sensor 124 can detect the pressure within the container 112 since the pressure within the gas supply line 132 will be the same as the gas pressure in the container, assuming fluid or make-up gas is flowing neither into nor out of the container 112. In one variation, when the container 112 is provided with a pressure detector connection, then the pressure sensor 124 can detect the pressure directly within the container itself. This variation is preferred, however many containers which are intended for use in the system 100 will not be provided with a pressure detector connection.

[0024] Another example of a sensor which can be used to detect an equilibrium condition within the container 112 is a temperature sensor 158, and the signal generated by the temperature sensor is a temperature signal. Preferably, the temperature sensor 158 is positioned in the container 112 so that it can detect the temperature of the liquid “L” within the container.

[0025] The system 100 also includes a controller 120. The controller can be, for example, a programmable logic controller (“PLC”) or a microprocessor. The controller is configured to receive the signal from the sensor(s) (e.g., a pressure signal from the pressure sensor 124, and/or a temperature signal from the temperature sensor 158). The controller 120 is further configured to determine when fluid is removed from the container 112, and, when the controller determines that fluid is removed from the container, to operate the source gas control valve 134 to allow the source gas 128 to flow to the container 112 to establish an equilibrium pressure within the container. Methods by which the controller 120 can determine that fluid has flowed out of the container 112 will be described further below. Preferably, as will also be explained further below, the equilibrium pressure which is reestablished in the container 112 by the controller 120 is a function of the signal(s) received by the controller 120 from the sensor(s) 124 and/or 158. The controller 120 can include a readable memory device 150 (such as a semiconductor memory chip) which can store pressure and/or temperature signals from the respective pressure sensor 124 and temperature sensor 158. Signals from the sensor(s) can be stored in the memory device as a function of time so that the controller 120 can determine changes in conditions within the container 112, as well as the instantaneous conditions within the container. The memory device 150 can also store a series of computer executable instructions (i.e., a “program”) 152 configured to be executed by the controller to perform the methods of the present invention, as will be explained further below. In one variation, described more fully below, the memory device 150 can further include a look-up table 156 of a plurality of determined equilibrium pressures as a function of temperature and/or pressure.

[0026] The system 100 can also include a user interface 160 which can include a display 162 which allows the controller 120 to communicate information (such as the detected pressure within the container 112, and/or the detected temperature within the container) to a user. The user interface 160 can further include a user input station, such as keypad 164, to allow a user to provide instructions to the controller 120.

[0027] In one variation, the system 100 includes a liquid outlet valve detector 130 which is configured to detect actuation of the liquid outlet valve 122 and to generate an outlet valve actuation signal in response thereto. The controller 120 can be configured to receive the outlet valve actuation signal, and in this instance the controller can be further configured to use the outlet valve actuation signal to determine that fluid has been removed from the container. That is, when the liquid outlet valve 122 is actuated to allow liquid “L” to flow from the container 112, the liquid outlet valve detector 130 will generate the outlet valve actuation signal, indicating that liquid “L” has been removed (or is being removed) from the container “L”. In this way the controller 120 will be able to determine that the equilibrium state in the container 112 has been altered, and will thus actuate the supply gas control valve 134 to allow the make-up gas 128 to flow into the container 112, thus reestablishing the equilibrium conditions which existed in the container 112 prior to the opening of the outlet valve 122.

[0028] In another variation, the system 100 can further include a source gas connection detector 148 which is configured to detect when the gas supply line 132 is connected to the container 112, and to generate a gas supply connection signal in response thereto. In this case, the controller 120 is further configured to receive the gas supply connection signal and to use the gas supply connection signal to determine that fluid (specifically, gas “G”) has been removed from the container. The source gas connection detector 148 is particularly useful when the container 112 is initially placed in service and connected to the gas supply line 132. That is, when the container 112 is initially connected to the gas supply line 132, a certain amount of the gas “G” in the container may flow out of the container 112 and into the gas supply line 132. The gas supply connection signal can cause the controller 120 to immediately acquire a pressure signal from the pressure sensor 124. This immediately acquired pressure signal will be a surge pressure when the gas “G” flows from the container 112 into the supply line 132. The controller 120 can then be configured to open the control valve 134 to allow the make-up gas 128 to flow into the container 112 to reestablish the pressure within the container to be the detected surge pressure. Preferably, the length of the gas supply line 132 is made as short as is practicably possible so that the surge pressure will be very near the pressure of the gas “G” in the container 112 prior to the container being connected to the gas supply line 132. When the pressure sensor 124 can be positioned in the container 112 itself, then the controller will be able to precisely determine the pressure of the gas “G” in the container prior to the gas “G” flowing into the supply line 132. In this case, the controller 120 will actuate the control valve 134 to allow the make-up gas 128 to flow into the container 112 until the pressure within the container is equal to the pressure detected prior to any gas “G” flowing from the container into the supply line 132.

[0029] Apparatus in accordance with the present invention (of which FIG. 2 presents but one example thereof) can be operated in various different manners. In a first example of operation, the controller 120 is configured to receive a pressure signal from the pressure sensor 124, and to use the pressure signal to determine when fluid is removed from the container. When the controller determines that fluid is removed from the container, the controller 120 operates the source gas control valve 134 to allow the source gas 128 to flow into the container 112 to reestablish the earlier equilibrium pressure within the container. Thus, the equilibrium pressure “EP” is a “function” of the first signal (pressure “P1”). Here, the function is “EP=P1”. This example is depicted in the time-pressure chart 300 of FIG. 4. The controller 120 (FIG. 2) continuously monitors the pressure “P” within the container as a function of time. By “continuously”, I mean continually at periodically predetermined time intervals. The predetermined time interval can be, for example, in milliseconds, in minutes, or any interval there between. At a first time “T1”, the spigot (liquid outlet valve) 122 (FIG. 2) is opened, and liquid “L” begins to flow from the container 112. At time “T2” (FIG. 4), the controller 120 (FIG. 2) determines that the gas pressure within the container 112 has dropped from the pressure “P1” (FIG. 4) to a lower pressure “P2”. This indicates to the controller 120 (FIG. 2) that fluid (specifically, liquid “L”) has been removed from the container 112. The controller 120 will then actuate the control valve 134 (at time “T2”) to allow the gas supply 128 to flow into the container 112. The controller 120 will allow the gas supply 128 to flow into the container 112 until the equilibrium pressure is reached. That is, the make-up gas 128 will be allowed to flow into the container 112 until the pressure in the container 112, as measured by the pressure sensor 124, is essentially equal to the pressure signal at a time “T1” (FIG. 4). In the example depicted, make-up gas will be added until at time “T3” the pressure within the container is essentially equal to “P1”. As can be seen by the graph 300 of FIG. 4, time “T1” is a time prior to a time “T2” when the controller 120 (FIG. 2) determines that fluid has been removed from the container.

[0030] Preferably, the controller 120 is configured to actuate the control valve 134 only when the controller determines that the difference in pressures “P1” and “P2” (FIG. 4) exceeds a predetermined amount. For example, the predetermined difference in pressures “P1” and “P2” can be selected to be 0.4 psig (approx. 0.03 kg/cm²). Thus, minor variations in the differential pressure (which can be the result of temperature changes within the container 112, or can result from shock and vibration experienced by the container 112) will not cause the controller 120 to actuate the control valve 134. It is only when the difference exceeds the preselected value will the controller 120 actuate the control valve 134. This will result in a more stable system for controlling the equilibrium pressure within the container 112. That is, by imposing a pressure differential limit on the controller 120 for actuating the control valve 134, the controller will be less prone to actuate the control valve 134 when the pressure differential is not the result of fluid being removed from the container 112.

[0031] More preferably, the controller 120 is configured to actuate the control valve 134 only when the controller determines that the difference in pressures “P1” and “P2” (FIG. 4) exceeds a predetermined amount over a predefined period of time. For example, with reference to FIG. 5, a chart 400 depicts the pressure “P” within a container (e.g., container 112 of FIG. 2) as a function of elapsed time “T”. In the example depicted in FIG. 5, the pressure “P” within the container gradually decreases between times “T0” and “T3”. This gradual decrease in pressure can be, for example, the result of a decreasing temperature in the container 122 (FIG. 2). For example, if the container 112 is a beverage keg which is initially stored at a first temperature, and is then put into service at a lower temperature, the temperature of the fluid within the keg will gradually drop over time. In order to prevent the controller from interpreting this temperature-related pressure drop as being a pressure drop resulting from fluid being removed from the container, a time interval “t” can be imposed on the controller. That is, the controller 120 can be configured to determine whether the drop in pressure “P” (FIG. 5) exceeds a predetermined amount over the time period “t”.

[0032] In this variation, when the controller detects that the reduction in the pressure signals exceeds the preselected value, the controller can determine the highest value of the stored pressure signals during the polling period. The controller can then use this highest value as the received pressure signal to reestablish the equilibrium pressure of the gas within the container. For example, with reference to FIG. 5, the pressure “P” within a container can gradually drop over time “T”, as indicated by the decreasing pressures P0, P1, P2, P3 and P3′. Within equal predetermined time periods (polling periods) t1, t2 and t3 (e.g., ten second time intervals), the corresponding decreases in pressure (i.e., P0 to P1, P1 to P2, and P2 to P3) do not exceed a predetermined limit (i.e., a preselected value, such as 0.4 psi). However, in the time period t4, the pressure in the container drops from P3 to P4. The controller 120 (FIG. 2) will detect that the pressure drop from P3 to P4 (FIG. 5) exceeds the predetermined limit, and will thus determine that the pressure drop is the result of fluid flowing out of the container 112 (FIG. 2). The controller 120 will then actuate the control valve 134 to allow the make-up gas 128 to flow into the container 112 (in the time interval t5, FIG. 5) until the detected pressure in the container is approximately equal to the highest detected pressure (i.e., P3) in the previous time interval t4. Although the newly established pressure P3 may not be precisely equal to the pressure P3′, by making the time interval “t” sufficiently small (for example, 10 seconds), the change in pressure resulting from a temperature change in the fluid over this time period will be negligible.

[0033] The controller 120 (FIG. 2) can be configured to acquire pressure information from the pressure sensor 124 a number of different times during the time interval “t” (FIG. 5). The pressure information can be stored in the memory device 150 (FIG. 2). For example, the controller can acquire pressure information every 0.5 seconds. Thus, the controller will be better able to detect a change in pressure which results from fluid flowing out of the container 112, thus more quickly reestablishing equilibrium in the container. When the interval “t” is 10 seconds, and the pressure is read by the controller every 0.5 seconds, then 20 pressure readings can be stored in the memory device 150. The oldest pressure reading can be replaced with the most recent pressure reading, such than a running ten-second window of pressure readings is always available for analysis.

[0034] In another variation on the present invention, the controller 120 (FIG. 2) can be configured to perform numerical analysis on recorded pressure values to more accurately determine the equilibrium pressure that should be reestablished in the container (112) once it is determined that fluid is flowing out of the container. With reference to FIG. 7, a chart 600 depicts pressure “P” within a container as a function of time “T”. As can be seen, initially (between pressure readings “P1” and “P5”) the pressure is gradually decreasing, as can result from cooling of the fluid in the container. The controller 120 can be configured to determine the rate of change of the pressure “P” as a function of time “T”. So long as the rate of change does not exceed a predetermined rate (e.g, 0.1 psi per minute), the controller 120 will not actuate the control valve 134 (FIG. 2). At a time between the time when pressure readings “P5” and “P6” are taken, the outlet valve (122, FIG. 2) is opened, and liquid begins to flow out of the container. The pressure reading “P6” may be insufficiently low to change the overall rate of pressure drop from the previous readings “P1” through “P5”, and so at the time reading “P6” is recorded, the controller cannot yet determined that the outlet valve has been actuated. However, when pressure reading “P7” is included in the numerical analysis of pressure readings, the controller will determine that the rate of change of pressure in the container has accelerated, and is now greater than the predetermined rate which will not cause the controller to actuate the control valve 134 (FIG. 2). Accordingly, the controller actuates the control valve to allow make-up gas (128, FIG. 2) to flow into the container (112, FIG. 2), as indicated by the change in the pressure curve at beginning at “P9” and ending at “P11” when the control valve is closed. The pressure continues to drop slightly at “P12”, due to cooling of the container.

[0035] The controller 120 (FIG. 2) can also use numerical analysis to determine the equilibrium pressure which should be reestablished in the container (112, FIG. 2) once it is determined that make-up gas (128, FIG. 2) should be added to the container. Again referring to FIG. 7, as described above, the controller will be able to determine that, at a time between the times when pressure readings P5 and P7 were recorded, the outlet valve (122, FIG. 2) was opened. The controller will then need to determine what the correct equilibrium pressure should be. Ideally, the equilibrium pressure is selected to be the pressure at the exact moment the outlet valve was opened. However, since no pressure reading was taken at this time, there is no reference to use. One solution is to use the last pressure reading that was recorded prior to determining that the outlet valve was opened. In this example, that would be pressure “P6” (since it was only after acquiring pressure reading “P7” that the controller was able to determine that the outlet valve had been opened). However, pressure reading “P6” may be lower than the theoretical equilibrium pressure (as indicated on the graph 600). Therefore, an alternate solution is to average the last pressure reading that was recorded prior to determining that the outlet valve was opened (i.e. “P6”) with the previously recorded pressure (“P5”). Further, this averaged pressure value can then be corrected for the known rate of change that existed prior to the outlet valve being opened (i.e., as established by the gradually decreasing pressure readings “P1” through “P5”. When the fluid pressure is constant prior to the outlet valve being opened (as depicted in FIG. 4), then the controller can determine that the equilibrium pressure is the steady-state pressure that was recorded prior to the controller determining that the outlet valve has been opened.

[0036] As mentioned previously, when the controller is configured to record (i.e., save in the memory device 150) a large number of pressure readings relatively close in time (e.g., every 0.1 second), then the controller will be better able to determine the correct equilibrium pressure to be reestablished within the container.

[0037] In the examples just described, the controller 120 uses a detected drop in pressure within the container 112 to determine that liquid has been (or is being) removed from the container 112. In another variation, when the system 100 of FIG. 2 is provided with a liquid outlet valve detector 130 (described above), then the controller 120 can be configured to receive the outlet valve actuation signal and to use this signal to determine that fluid has been (or is being) removed from the container. The controller 120 can then use the pressure signal which it last received prior to determining that fluid has been removed from the container as the equilibrium pressure. The controller 120 then actuates the control valve 134 to allow make-up gas 128 to flow into the container 112 until the pressure in the container is essentially equal to the last-determined equilibrium pressure. The use of a liquid outlet valve detector simplifies somewhat the process of determining the correct equilibrium pressure to be reestablished in the container, since the controller can be configured to take an instantaneous pressure reading as soon as the outlet valve actuation signal is received. Further, the liquid outlet valve detector 130 can be configured to generate the outlet valve actuation signal prior to the outlet valve 122 actually allowing any fluid to flow out of the container. This helps to ensure that the pressure reading acquired by the controller 120 upon receipt of the outlet valve actuation signal will not be affected by liquid flowing out of the container 112 (as was the problem describe above with respect to FIG. 7)

[0038] In another variation, when a temperature sensor 158 is used to detect the temperature of the liquid “L” in the container 112, then the controller 120 can include a look-up table 156 (which can be stored in the memory device 150). An example of one such look-up table is depicted in FIG. 6. The look-up table 500 of FIG. 6 includes a listing of equilibrium pressures “EP1” through “EP4” for a first gas GAS1, “EP5” through “EP8” for a second gas GAS2, “EP9” through “EP12” for a third gas GAS3, and “EP13” through “EP16” for a fourth gas GAS4. A user can select the gas for which the table will be used, for example by using the keypad 164 (FIG. 2). However, in some instances (such as beverage dispensing) the gas will always be the same, and no such selection will be required. The look-up table 500 further includes a list of temperatures, shown here as 36° F., 38° F., 40° F. and 42° F. Once the controller 120 has determined that fluid has been removed from the container 112 (in any of the methods described above), then the controller can measure the temperature (or use the last measured temperature, which can be stored in the memory device 150) of the liquid in the container, and can then look-up the appropriate equilibrium pressure “EP” from the look-up table 500, knowing the temperature and gas as cross references. The controller 120 (FIG. 2) can then actuate the control valve 134 to allow make-up gas 128 to flow into the container 112 to reestablish the equilibrium which existed prior to fluid being removed from the container.

[0039] In one variation on the look-up table 500 of FIG. 6, rather than being based on gasses GAS1 through GAS4, the look-up table can instead be based on the liquid in the container, so that the list of gasses are replaced with a list of different types of liquids. For example, in a beverage dispensing environment, the various potential liquids can include, water, soft drinks, and two different types of beer. Thus, the level of carbonation in each beverage can be different, as reflected by the look-up table. For example, soda water will typically have a much higher level of carbonation than beer, and accordingly the equilibrium pressures listed in the look-up table for soda water will be higher than the equilibrium pressures listed for the two types of beer.

[0040] As indicated in FIG. 2, the system 100 can include a series of executable instructions in the form of a program 152. The series of executable instructions are configured to be executed by the controller 120 to determine the equilibrium pressure to be achieved, as well as to perform other operational aspects of the system. FIG. 3 depicts an exemplary flowchart 200 of executable steps which can comprise the program 152 of FIG. 2. I will discuss FIG. 3 with reference to the exemplary system 100 depicted in FIG. 2. With reference to FIG. 3, at step 202 the controller 120 (FIG. 2) determines whether a container 112 has been connected to the system (i.e., at the supply-gas connection 144). (In the exemplary flowchart, the container is referred to as a “keg”, but this should not be considered as restricting the type of container that can be used.) The controller 120 can make the determination of whether or not the keg has been connected in at least two different manners, described above. That is, the system can include a source gas connection detector 148, or alternately, the controller can check for a pressure surge in the supply gas line 132 when the gas “G” in the container 112 initially flows into the line 132 after the container has been connected to the line. Another way in which the connection can be ascertained is by an instruction from a user via the user interface keypad 164. At step 202 of the flowchart, if the controller 120 determines that a container (keg) has not been connected to the system, the controller continues to poll at step 202 for a connection.

[0041] However, if at step 202 the controller 120 does determine that a container has been connected to the system, then at step 204 the controller determines whether the gas pressure in the container is the correct pressure for the desired equilibrium condition. This can be accomplished as described above by detecting the surge pressure within the line 32. If the pressure drops by more than a preselected amount after the container is connected to the line, then pressure can be considered too low, or “incorrect”. Alternately, the controller can use a look-up table in the manner described above with respect to FIG. 6 to determine whether the pressure in the container is “correct”. If at step 204 the controller 120 determines that the pressure is not “correct”, the controller can actuate the control valve 134 to add supply gas 128 to the container 112 until the desired equilibrium pressure is established.

[0042] If the pressure in the container is determined at step 204 to be “correct”, then no make-up gas 128 needs to be added to the container at this time, and at step 208 the controller monitors the pressure in the container to determine if liquid has been removed from the container (as for example, by activation of outlet valve (“faucet”) 122). Examples of how the container pressure can be monitored were described above with respect to FIGS. 4, 5 and 7. Further, if the system is provided with a liquid outlet valve detector 130 (described above), then the controller will be able to directly detect outlet valve actuation. However, even when a liquid outlet valve detector 130 is employed, the controller will still monitor the pressure, and store the most recent pressure readings in the memory device 150 (FIG. 2), so that the controller will be able to determine what the correct equilibrium pressure is that is to be reestablished in the container. (As described above, alternately a temperature sensor 158 and a look-up table can be used to determine the correct equilibrium pressure.)

[0043] If no outlet valve actuation is detected at step 208 of FIG. 3, then at step 212 the controller 120 (FIG. 2) checks to determine whether the container is still connected to the system. This determination can be made in a number of different manners, including by using a source gas connection detector 148, by user instruction via the keypad 164, or, less preferably, by detecting no pressure reading in the container. If at step 212 the container has been determined to have been disconnected from the system, then the controller reverts to step 202 to check for a container to be connected to the system. However, if at step 212 the controller determines that the container has not been disconnected, then the controller returns to step 208 to check for actuation of the outlet valve 122, indicating that liquid “L” has been removed from the container 112, as described above.

[0044] Once outlet valve actuation is detected at step 208, then at step 210 the controller 120 actuates the control valve 134 to allow the pressurized gas source 128 to add make-up gas to the container 112, thus bringing the container back into the equilibrium state (or near to the state) which existed prior to liquid being removed from the container. I have described above various methods and apparatus for correctly determining the amount of make-up gas to be added to the container to reestablish the equilibrium conditions.

[0045] The invention further includes a method of maintaining an equilibrium pressure between a gas and a liquid within a container. As explained earlier, the equilibrium pressure is “maintained” in the overall sense in that it is reestablished after it falls below a desired level. The method includes providing a container (such as container 112, FIG. 2) having a fluid comprising a liquid “L” and a gas “G” in equilibrium, and determining when fluid is removed from the container. (Methods and apparatus for determining when fluid has been removed from the container have been described above.) A quantity of a make-up gas (128) is added to the container 112 when it has been determined that fluid has been removed from the container. The method can further include determining a pressure of the gas in the container at a first time (e.g., pressure “P1” at time “T1”, as in FIG. 4). This “first time” is prior to a later time (e.g., time “T2”, FIG. 4) when it has been determined that fluid has been removed from the container. The make-up gas is then added to the container until the pressure of the gas in the container is approximately equal to the pressure of the gas at the first time. For example, in FIG. 4 make up gas is added between times “T2” and “T3” until the pressure at time “T3” is approximately equal to pressure “P1”.

[0046] The step of determining when fluid has been removed from the container can be performed when the container is connected to a gas supply line (e.g., step 202 of flowchart 200, FIG. 3), as well as when a portion of the liquid flows out of the container (e.g., step 208 of flowchart 200).

[0047] The method can optionally include the step of determining a temperature of the liquid in the container at a first time, the “first time” being prior to a time when it has been determined that fluid has been removed from the container. The quantity of the makeup gas to be added to the container can then be determined as a function of the first temperature. For example, by using the look-up table 500 of FIG. 6, the controller 120 (FIG. 2) can determine that make-up gas needs to be added to the container until the designated equilibrium pressure (“EP”) is reached. Thus, it is not so much the volume of make-up gas which is determined using the look-up table, but rather a target pressure to be reached as a result of adding the make-up gas to the container.

[0048] In yet another embodiment of the present invention, a method of maintaining an equilibrium pressure between a gas and a liquid within a container includes providing a container (e.g., 112, FIG. 2) having a fluid comprising a liquid and a gas in equilibrium, and determining when fluid is removed from the container. (These first two steps are the same as the first two steps of the method previously described.) The method further includes measuring a pressure of the gas in the container at about the time it has been determined that fluid has been removed from the container. For example, pressure is measured at P3 and P3′ during the interval “t4” in FIG. 5. A make-up gas (such as 128, FIG. 2) is then added to the container until the pressure in the container is approximately equal to the (previously) measured pressure (e.g., P3, P3′, or any intermediate pressure there-between, FIG. 5).

[0049] Preferably, the pressure of the gas (“G”, FIG. 2) is measured in the container at a first time (e.g., time “T3”, FIG. 5) which is prior to the time it has been determined that fluid has been removed from the container (e.g., time T3′). In one variation, the pressure of the gas (“G”, FIG. 2) is measured in the container (112, FIG. 2) a second time (e.g., time “T4”, FIG. 5), the second time being after the first time (e.g., after “T3”). A difference is then determined between the pressure at the first time (e.g., P3 at T3) and the pressure at the second time (e.g., P4 at T4). Then, when the difference exceeds a preselected amount, make-up gas is added to the container. As described previously, the second time (“T4”) can be selected to be not more than about ten seconds after the first time (“T3”) so that equilibrium pressure corrections can be made in a timely manner.

[0050] While the above invention has been described in language more or less specific as to structural and methodical features, it is to be understood, however, that the invention is not limited to the specific features shown and described, since the means herein disclosed comprise preferred forms of putting the invention into effect. The invention is, therefore, claimed in any of its forms or modifications within the proper scope of the appended claims appropriately interpreted in accordance with the doctrine of equivalents. 

I claim:
 1. A system for maintaining a gas in equilibrium with a liquid stored in a container with the gas, comprising: a sensor configured to detect an equilibrium condition within the container and to generate a first signal in response thereto; a source gas connection configured to allow a pressurized source gas to flow to the container; a source gas control valve configured to control the flow of the source gas to the container; and a controller configured to receive the first signal, to determine when fluid is removed from the container, and, when the controller determines that fluid is removed from the container, to operate the source gas control valve to allow the source gas to flow to the container to establish an equilibrium pressure within the container, the equilibrium pressure being a function of the first signal.
 2. The system claim 1, and wherein the sensor is a pressure sensor, the first signal is a pressure signal, and the equilibrium pressure is essentially equal to the pressure signal at a time prior to a time when the controller determines that fluid has been removed from the container.
 3. The system claim 1, and wherein: the controller comprises a look-up table of a plurality of determined equilibrium pressures as a function of temperature; and the sensor is a temperature sensor, the first signal is a temperature signal, and the equilibrium pressure is established as the determined equilibrium pressure which corresponds to the temperature signal.
 4. The system claim 3, and further comprising a pressure sensor configured to determine gas pressure within the container and to generate a pressure signal in response thereto, and wherein the plurality of determined equilibrium pressures is further a function of pressure, and the equilibrium pressure is further established as the determined equilibrium pressure which corresponds to the pressure signal.
 5. The system claim 1, and wherein the controller comprises a readable memory device containing a series of executable instructions configured to be executed by the controller to determine the equilibrium pressure as a function of the first signal.
 6. The system claim 5, and wherein the series of executable instructions are further configured to be executed by the controller to calculate an equilibrium pressure when a container is placed in an initial fluid communication with the source gas connection.
 7. A system for maintaining a gas in equilibrium with a liquid stored in a container with the gas, comprising: a pressure detector configured to detect pressure of the gas within the container and to generate a pressure signal in response thereto; a gas supply line defined by a first end configured to be placed in fluid communication with a pressurized source gas, and a second end configured to be connected to the container; a source gas control valve disposed within the gas supply line; and a controller configured to receive the pressure signal, to determine when fluid is removed from the container, and, when the controller determines that fluid is removed from the container, to operate the source gas control valve to establish the pressure of the gas within the container to essentially the same pressure as the received pressure signal at a time prior to determining that fluid has been removed from the container.
 8. The system of claim 7, and wherein the pressure detector is connected to the gas supply line between the gas control valve and the second end of the gas supply line.
 9. The system claim 7, and wherein, when the container is configured to be connected to a liquid outlet valve to allow liquid to be removed from the container, the system further comprising a liquid outlet valve detector configured to detect actuation of the liquid outlet valve and to generate an outlet valve actuation signal in response thereto, and wherein the controller is further configured to receive the outlet valve actuation signal and to use the outlet valve actuation signal to determine that fluid has been removed from the container.
 10. The system claim 7, and further comprising a source gas connection detector configured to detect when the gas supply line is connected to a container, and to generate a gas supply connection signal in response thereto, and wherein the controller is further configured to receive the gas supply connection signal and to use the gas supply connection signal to determine that fluid has been removed from the container.
 11. The system claim 7, and wherein the controller comprises a readable memory device configured to store a plurality of pressure signals generated by the pressure detector.
 12. The system claim 11, and wherein the controller is further configured to: store the plurality of pressure signals in the memory device as a function of time; and detect a reduction in the pressure signals over a selected period of time, and to determine that fluid has been removed from the container when the reduction in the pressure signals exceeds a preselected value.
 13. The system claim 12, and wherein the controller is further configured to determine, when the controller detects that the reduction in the pressure signals exceeds the preselected value, the highest value of the stored pressure signals, and to use the highest value as the received pressure signal to establish the pressure of the gas within the container.
 14. A method of maintaining an equilibrium pressure between a gas and a liquid within a container, comprising: providing a container having a fluid comprising a liquid and a gas in equilibrium; determining when fluid is removed from the container; and adding a quantity of a make-up gas to the container when it has been determined that fluid has been removed from the container.
 15. The method of claim 14, and further comprising: determining a pressure of the gas in the container at a first time, the first time being prior to a time when it has been determined that fluid has been removed from the container; and wherein the make-up gas is added to the container until the pressure of the gas in the container is approximately equal to the pressure of the gas at the first time.
 16. The method of claim 14, and wherein determining when fluid is removed from the container is performed when the container is connected to a gas supply line.
 17. The method of claim 14, and wherein determining when fluid is removed from the container is performed when a portion of the liquid flows out of the container.
 18. The method of claim 14, and further comprising: determining a temperature of the liquid in the container at a first time, the first time being prior to a time when it has been determined that fluid has been removed from the container; and determining the quantity of the make-up gas to add to the container as a function of the first temperature.
 19. A method of maintaining an equilibrium pressure between a gas and a liquid within a container, comprising: providing a container having a fluid comprising a liquid and a gas in equilibrium; determining when fluid is removed from the container; measuring a pressure of the gas in the container at about the time it has been determined that fluid has been removed from the container; and adding a make-up gas to the container until the pressure in the container is approximately equal to the measured pressure.
 20. The method of claim 19, and wherein the pressure of the gas is measured in the container at a first time prior to the time it has been determined that fluid has been removed from the container.
 21. The method of claim 19, and wherein: the pressure of the gas is measured in the container at a first time prior to the time it has been determined that fluid has been removed from the container; the pressure of the gas is measured in the container at a second time, the second time being after the first time; determining a difference between the pressure at the first time and the pressure at the second time; and when the difference exceeds a preselected amount, adding the make-up gas to the container.
 22. The method of claim 21, and wherein the second time is selected to be not more than ten seconds after the first time. 